Sequential Plasma-Activated Bonding Mechanism of Silicon/Silicon Wafers

To investigate the sequentially plasma-activated bonding (SPAB) mechanism of silicon/silicon wafers, the surface hydrophilicity, and the interface voids, nanostructures and chemical compositions that control the bonding quality, such as bonding strength, have been observed. Although the sequentially plasma-activated surfaces are hydrophilic, the SPAB mechanism is not identical to the hydrophilic bonding. SPAB shows high bonding strength at room temperature and water rearrangement below 150 degrees C, which removes the water from the interface to the bulk. This results in a thinner amorphous silicon oxide layer at the interface. Further heating of the bonded wafers desorbs water from the bulk. The heating at 225 degrees C starts producing hillocks at the interface, which turn into voids at temperatures above 400 degrees C for absorbing the hydrogen gas produced from the desorbed water at the interface. The new and bigger voids are due to the hydrogen gas at 600 degrees C and start accumulating at 800 degrees C, resulting in bubbles caused by the accumulation of voids at the preferential sites. No nitrogen exists either in silicon or in the amorphous SiO2 layer at the interface. The Si-L-2,L-3 edges from the amorphous silicon oxide at the bonded interface are identical to those of the standard SiO2. [2009-0253]